This invention relates to printed wiring boards (PWBs) with a copper/dielectric/aluminum structure, and, more particularly, to the processing of such PWBs to produce plated holes therein.
Copper-clad laminates with an aluminum backing are patterned with electrical circuitry and used in a number of applications such as high-frequency microwave circuits. Such laminates, termed printed wiring boards or PWBs, include an aluminum backing, a layer of a dielectric such as a resin overlying the aluminum backing, and a patterned copper layer overlying the dielectric layer. It is often necessary to provide holes through the copper layer and the dielectric layer, and into the aluminum backing. The bores of the holes are plated with an electrical conductor so that they provide electrical continuity between the copper layer and the aluminum backing. The holes may extend through the aluminum backing or may be blind holes terminating within the aluminum backing.
The uniform plating of the bores with sufficient thickness, and in an economical manner, is difficult, for several reasons. The holes are long and narrow, making difficult the uniform plating of the bore with minimal defects. Bubbles may become trapped within the bores and physically block the subsequent plating. Copper and aluminum are chemically dissimilar. Chemicals used to process copper, such as etchants, often attack aluminum, and vice versa.
A number of techniques have been used to accomplish the plating of the bores in conventional low-frequency (less than 500 MHZ) PWBs. Since these low-frequency PWBs have copper layers on both the top and bottom sides, the plating of the bores is relatively simple. In industry, electroless copper plating or direct metallization is used to provide electrical continuity between the top and bottom sides. When there is an aluminum backing, however, the plating of the bores is difficult without proper processing sequences and techniques. In one approach, the conductive copper layer is applied by a physical technique such as physical vapor deposition. This approach provides an adherent copper coating on the readily exposed surface of the bore, but uniform deposition of copper inside deep holes is difficult, with the result that the electrical path between the copper and aluminum may be unreliable.
There is a need for an improved approach to the production of aluminum-backed PWBs having blind or through holes therein. The present invention fulfills this need, and further provides related advantages.